A novel experimental method was developed to study negative physical
and chemical effects of latex and cardiac glycosides on first-instar
monarch butterfly larvae in their natural environment in north central
Florida. Forceps were used to nibble through the petioles of leaves
of the sandhill milkweed Asclepias humistrata, mimicking the behaviour
of mature monarch larvae. This notching cut off the supply of latex
to the leaves without significantly reducing either their cardiac glycoside
concentration or water content.

The mean cardiac glycoside concentration in larvae that fed on intact
leaves was nearly two and half times greater than in those that fed
on notched leaves. This was probably because more latex is present in
the gut of the larvae that fed on the intact leaves. Supporting this
is the fact that the mean concentration of cardiac glycosides in the
latex was 34 to 47 times that in the leaves.

Wet weights, dry weights and growth rates of first-instar larvae that
fed on intact leaves over a 72-h period were less than half those that
fed on notched leaves.

Mortality due to miring in the latex was 27% on the intact leaves compared
with 2% on the notched leaves.

Latex, cardiac glycosides, and other as yet undetermined plant factors
all have a negative effect on first-instar larval survival.

Videoanalyses indicated that ingestion of latex caused the larvae to
become cataleptic and increased their chances of being mired on the
leaf by the setting latex glue. Dysfunction resulting from latex ingestion
may lead to the larvae falling off the plant and being killed by invertebrate
predators.

The difficulty of neonate monarch larvae surviving on A. humistrata
- one of the principal milkweed species fed on each spring as monarchs
remigrate from Mexico into the southern U.S.A. - is evidence that a
coevolutionary arms race is operating in this plant-herbivore system.